“Once in a generation” is just a catchphrase though, there’s no binding agreement that means Scotland needs to wait 20 years between votes. If the result next year is Independence then surely it was the right time to hold a vote as enough time had past that a considerable portion of the population had changed their mind and wanted change.
As for a vote on the Senedd, there was an Abolish the Welsh Assembly party that won no seats and got a tiny percentage of the vote, ~3% if I recall.